This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
This invention relates to mobile offshore units, specifically self-elevating units. Self-elevating units are used in the offshore industry for a multitude of tasks including but not limited to drilling and production operations, general construction operations, crew accommodation, wind-turbine installation, etc. Self-elevating units can refer to jackups, liftboats, jackup barges, mobile offshore drilling units (MODUs), mobile offshore production units (MOPUs), or the like.
Self-elevating units (henceforth referred to as “units”) are typically constructed of a hull, supported on one or more legs which extend through or on the side of the hull. A lifting system or “jacking system” is installed on the unit for the purpose of raising or lowering the legs relative to the hull. The self-elevating unit is designed such that the hull is buoyant and can float, supporting itself and the legs and its cargo (e.g., in an “afloat” mode). Once the self-elevating unit reaches the desired location where it is to operate, the legs are lowered to the seabed and the hull is raised above the waterline (e.g., in an “elevated” mode), so that there is no longer a buoyancy force on the hull, creating a stable platform with a positive airgap. One of the first steps in the process of transitioning from afloat to elevated mode is commonly referred to as “going on location.”
Conversely, the process of transitioning from elevated mode to afloat mode is commonly referred to as “coming off location.” The self-elevating unit lowers its hull from positive air gap into the water, partially submerging the hull. The jacking system lowers the hull until the buoyancy of the hull is sufficient to extract and raise the legs. The legs of the self-elevating unit typically incorporate a footing that provides the bearing or contact surface between the seabed (and/or the soil beneath the seabed) and the unit. The legs of a self-elevating unit may have an individual footing for each leg (spudcan) or the legs may share a common footing (mat).
Analysis of units in the elevated and afloat modes is fairly well understood, and standards exist for use in the development of proper operating conditions for both modes of operation. The transition phase for self-elevating units (e.g., going on location or coming off location), however, is not as well understood.